Method for controlling the formation of polymer accumulations during distillation of a vinylaromatic monomer

Abstract

A method for controlling the formation of polymer accumulations in a distillation column comprising accumulating liquid phase material containing a polymerization inhibitor in a seal pan; providing the seal pan with liquid-pervious weep hole means through the bottom of the pan and allowing a controlled quantity of polymerization inhibitor-containing liquid phase material to seep through the weep hole means to the underside of the seal pan; and apparatus useful for practicing the disclosed method.

We claim: 
     
       1. A method for controlling the formation of polymer accumulations on the underside of a seal pan underneath a downcomer in a downcomer-equipped distillation column used for distillative purification of a vinylaromatic monomer, said method comprising: accumulating liquid phase material containing an effective polymerization inhibiting concentration of polymerization inhibitor in the seal pan; and   providing liquid pervious weep hole means through said seal pan spaced and dimensioned to provide a controlled quantity of polymerization inhibitor-containing liquid phase material from the seal pan through said weep hole means to the underside of the seal pan.   
     
     
       2. A method according to claim 1 wherein said distillation is effected at subatmospheric pressure. 
     
     
       3. A method according to claim 2 wherein said distillation is effected at a temperature from about 65 degrees C. to about 138 degrees C. and an absolute pressure from about 0.013 atmosphere to about 0.26 atmosphere. 
     
     
       4. A method according to claim 3 wherein said temperature lies in the range from about 90 degrees C. to about 115 degrees C. 
     
     
       5. A method according to claim 1 wherein said distillation takes place in the presence of a polymerization inhibitor. 
     
     
       6. A method according to claim 5 wherein said inhibitor comprises at least one compound selected from the group consisting of dinitro-o-cresol, dinitro-p-cresol, m-nitro-p-cresol, dinitrophenol, N-nitroso-diphenylamine, 4-halo-3,5-dinitrotoluene, and 3-nitro-2,5-cresotic acid. 
     
     
       7. Method according to claim 5 wherein said inhibitor is present in an amount from about 50 to about 3000 parts per million with respect to said vinylaromatic monomer. 
     
     
       8. Method according to claim 7 wherein said inhibitor is present in an amount from about 200 to about 1000 parts per million with respect to said vinylaromatic monomer. 
     
     
       9. Method according to claim 5 wherein said additional inhibitor comprises a mixture of a first inhibitor substance active primarily in the vapor phase and a second inhibitor substance active primarily in the liquid phase in said distillation. 
     
     
       10. Method according to claim 5 wherein said inhibitor comprises a mixture of dinitro-p-cresol and N-nitroso-diphenylamine. 
     
     
       11. Method according to claim 5 wherein said inhibitor is introduced to said distillation in admixture with the crude feed. 
     
     
       12. Method according to claim 5 wherein said inhibitor is introduced directly into at least one distillation column in which said distillation takes place. 
     
     
       13. Method according to claim 5 wherein said inhibitor is introduced into a reboiler used to maintain a distillation column used for said distillation at distillation temperature. 
     
     
       14. A method according to claim 1 wherein said weep hole means comprises a plurality of weep holes formed at spaced intervals through the bottom of said seal pan. 
     
     
       15. A method according to claim 14 wherein each weep hole is a circular aperture having a diameter from about 2 to about 10 millimeters. 
     
     
       16. A method according to claim 15 wherein the diameter for each weep hole is from about 4 to about 8 millimeters. 
     
     
       17. A method according to claim 16 wherein the distance between adjacent weep holes is from about 4 to about 25 times the diameter of the holes. 
     
     
       18. Method according to claim 1 wherein said vinylaromatic monomer is selected from the group consisting of styrene, vinyltoluene, alpha-alkylstyrene, and divinylbenzene. 
     
     
       19. Method according to claim 18 wherein said vinylaromatic monomer is vinyltoluene. 
     
     
       20. Method according to claim 18 wherein said vinylaromatic monomer is styrene. 
     
     
       21. A method for the distillative purification of a vinylaromatic monomer comprising the steps of: (a) distilling said crude vinylaromatic monomer to separate a recycle overhead fraction comprising lower boiling materials from a recycle bottoms fraction comprising vinylaromatic monomer and higher boiling materials,   (b) distilling said recycle bottoms fraction to separate a finish overhead fraction comprising substantially pure vinylaromatic monomer from a finish bottoms fraction comprising higher boiling materials, and   (c) providing liquid pervious weep hole means through the seal pans of a distillation column used for at least one of said distilling steps spaced and dimensioned to provide a controlled quantity of inhibitor-containing liquid phase material through said weep hole means, wherein the formation of polymer accumulations on the underside of seal pans underneath the downcomers in downcomer-equipped distillation columns used for said distillative purification is controlled by accumulating liquid phase material containing an effective polymerization inhibiting concentration of polymerization inhibitor in said seal pans and allowing a controlled quantity of polymerization inhibitor-containing liquid phase material from said seal pans to seep through said weep hole means to the underside of said seal pans.     
     
     
       22. A method according to claim 21 further comprising distilling said recycle overhead fraction to separate a bottoms fraction of alkylbenzene precursor for said vinylaromatic monomer from an overhead fraction comprising other lower boiling materials, and feeding said alkylbenzene bottoms fraction to a dehydrogenation reactor for producing additional crude vinylaromatic monomer feed.